Paper coating material and the paper coated therewith



Patented Feb. 18, 1941 PATENT. o FlcE PAPER COATING MATERIAL AND THE PAPER COATED THEBEWITH James S. Oiiutt, Evanston, and Joseph W. Gill,

Elmhnrst assixnors to United States" Gypsum bompany, Chicago, 111., a corporation of Illinois No Drawing.

Application March 2, 1939,

Serial No. 259,386

I Claims.

This invention relates to paper coating material and the paper coated therewith.

This application is a continuation in part of co-pending application Serial No. 666,352, flied v 6 April 15, 1933.

In the coating of paper, it has been the practice to use satin white as the pigment base of the coating material. Satin white is made by treating an alum solution with lime so that the resulting product contains a substantial proportion of aluminous gel, which gives the coating material certain characteristics that are objectionable. The use of pure recrystallized and deflocculated gypsum, free of aluminous gel, instead of satin white, gives a coating which, although acquiring a somewhat lower (less glossy) finish on calendering than satin white coatings, will show excellent levelness or smoothness. which is necessary to permit high grade printing. The coating which can be produced from our special form of gypsum also takes ink exceptionally well and in addition has other important advantages. A coating comprising the 25 improved deflocculated gypsum base shows less tendency to darken when heavily calendered than does a coating material with a satin white base. The deflocculated gypsum requires less casein to secure a given adhesion of the coatin 30 to the body of the paper than that required when satin white is used as a base pigment.

The deflocculated gypsum'coating material can be dried, disintegrated, and then readily mixed.

again with water to form a smooth slurry. which 5 is not true with satin white finishes, which are not'readily re-dispersed properly after drying, owing to the presence'of partially dehydrated aluminum oxide which is not reversible to gel s cm.

In hydrating piasteroi Paris to form deilocculated hydrated gypsum, it has heretofore been thought necessary to use a relatively large amount of water (from 4 to 8 parts water P part plaster by weight) to prevent the slurry from setting during the agitation when a paddle agitator is used. The use of this large amount of water at a paper coating mill would be a considerable inconvenience, as it necessitates a P r ial dewatering of the prepared re- 50 crystallized gypsum slurr before it can be mixed 1 sary to remove the excess water. However, while we have found it advantageous to prepare the paper coating with a much smaller amount of water than above indicated, yet, if desired, the deflocculated gypsum may be prepared 6 by the use of a large amount of water to produce substantially the same end product; for we behave the use oi' recrystalllzed and/or defloccu lated gypsum as a paper coating material to be new regardless of its method of manufacture;

We have found that by adding certain electrolytes in" small amounts to the. water in which the plaster is hydrated to form deflocculated gypsum, the tendency for the slurry to stiffen excessively or set during the hydration while 15 being stirred in any suitable agitator, is consid erably reduced, thus permitting the manufacture of deflocculated gypsum from a mixture of 1 to 2 parts water per part of plaster by weight.

A deflocculated gypsum so formed can be compounded to a satisfactory coating color by merely adding the requisite quantity of casein glue. We have found that from 1 to 2 parts by weight of sodium carbonate per 100 parts of calcined gypsum plaster, or an equivalent amount of sodium sulphate, are effective in reduclng the setting tendency of the plaster, although these limits can be widened considerably without essentially changing the characteristics desired in the process. A" still more effective electrolyte is sodium phthalate, especially when made alkaline with a slight excess of caustic soda. The potassium salts of the above named electrolytes may also be used, if desired. Cer-' tain features of the herein describedprocess oihydrating calcined gypsum are disclosed and claimed in co-pending Joseph W. Gill application Serial No. 107,371 filed October 24, 1936, which-is a division of Ofliitt and Gill application Serial No. 666,352 flied April .15, 1933.

The exact reason for this action of these electrolytes is not completely known. Their efiect onthe size and shape of the gypsum crystals obtained on hydrating the calcined gypsum plasterswill, undoubtedly, afi'ord a partial explanation of the action. The crystals of hydrated gypsum, particularly when the alkaline phi' hal- 'ateis used, have a smaller length to breadth ratio than when ordinary plaster-water inix or plaster water mix containing small amounts of othersalts are used. In generaL; as the particle size; decreases, the consistency, the casein required for coating material; the opacity or hiding power, and hence, the whiteness of the applied coating, and the levelness or printing smoothness of the coating, increase. The glare obtainable seems to vary but little with change in particle size, but can be varied considerably by varying the calendering process. An additional advantage of the controlled shape and size obtainable with our improved gypsum pigment lies in. the improved spreading qualities obtainable, for the spreading characteristics of precipitated gypsum. color depends somewhat on the ratio of crystal length to thickness, an excessive length generally causing poorer spreading characteristics. I

The product resulting when the calcined gypsum is agitated with the above amount of water, containing the electrolyte, consists of a slurry having about the consistency of stifi cream.

.This fluid slurry, after about one hour of agitation, contains defiocculated, completely hydrated gypsum in recrystallized form, the crystals averaging 4 times as long as broad with an average crystal length of 5.4 microns, while gypsum similarly hydrated but in the presence of water alone has crystals measuring about times as long as broad and an average crystal length of 14 microns. A coating composition suitable for-manyv purposes can be prepared by simply adding the proper amount of adhesive, such as casein glue, to the slurry. However, in many other cases it will be found desirable to incorporate other materials well known to the art, along with the adhesive and our improved gypsum pigment, to secure a coating compositio having the properties desired.

This new gypsum product having the unique properties above described, will be termed gamma gypsum" in the claims in order to differentiate from the gypsum products known to the prior art.

As a specific illustration of the application of this invention in the preparation of a caseincontaining coating color, the following formula has been used with satisfaction: 8.5 parts by weight phthalic acid, and 5.5 parts of sodium hydroxide were dissolved in 1300 parts of water at room temperature. 850 parts of white molding plaster or calcined gypsum were added to this solution and the mix was stirred i'or one hour in a standard mechanical mixer. The slurry produced was now a viscous fluid. To this slurry were added 1100 parts of casein glue containing 170 parts of dry casein. It is immaterial whether the sodium phthalate is added as such, or whether it is formed in the solution by the use of phthalic acid or anhydride, together with the equivalent amount of sodium hydroxide, a slight excess of the latter being desirable. The resulting mixture after straining represents the coating color which is used directly on the paper coating machine.

It is sometimes desirable to utilize a mixture of the deflocculated gypsum and coating clay; In making up such a coating color, utilizing sodium carbonate as the electrolyte, the following procedure is followed:

To the agitator, 130 lbs. (15% gallons) of water at a temperature 01"55" to F. are added. One

pound of soda ash is dissolved in this water,

about 68 pounds of the calcined gypsum molding plaster are added to the solution and violent agitation started and continued for about. 20 minutes, until the plaster slurry has stifiened, and the agitation is continued for 8 to 10- minutes thereafter. During the stiffening, the agitator should be operated as vigorously as the equipment will permit. The total time of agitation required will vary from 30 to 60 minutes, depending on slight variations in temperature, character of water used, and the nature of the'agitator. During this agitation, the plaster will hydrate, taking up 12 pounds of water to form pounds of inhibit any tendencies which the mixture may exhibit toward corrosion of iron utensils.

Immediately before adding the casein glue, dissolve 2 /2 pounds of soda ash in '12 pounds of water (heating gently if necessary to effect solu; tion) and stir this into the hydrated gypsum-g clay slurry. Then add the requisite amount of cool (not over F.) casein glue. One hundred and eight pounds of casein glue of the formula given below contain about 1'7 pounds of dry casein, which is the average requirement for the batch of coating color described. After straining the color through a power strainer, a pint of Turkey red oil is added. The coating color is then ready for use, but may be thinned with water if desired. Although the adherence between the paper coating and the paper varies somewhat with the method used in preparing the casein glue, any of the ordinary casein solvents may be used without trouble. The following formula for the casein glueand the process of cutting has given good results:

Pounds .water n n' 250 Casein 50 Soda Ash 10 Bor 2 The casein is soaked with about A of the water for 30 minutes, stirring occasionally. The solvents are dissolved in the balance of the water and mixed with the casein after the soaking.

The glue isv then heated in a steam or hot water jacketed kettle to about F. for 10 minutes, taking care not to overheat the glue above F. Before mixing with the precipitated ypsum slurry, the glue should be cooled below 110 F. or curdling of the casein is apt to follow.

If desired, wax or stearic acid emulsions may be to inhibit any scufling tendency which the paper cure a uniform dispersion. A stearic acid emulsion may be prepared asiollows:

" Y Pounds,

Stearic acid so Water 15o Borax i /2 Theabove mixture should be heated to complete solution and l50'pounds more of hot water then added, stirring until a smooth, creamy emulsion is obtained. When a waterproof coating is desired, a dilute solution of formaldehyde containing 0.5 to 2.0% formaldehydebased on the casen,

should be added to the slurry along with the casein glue. Animal glue, starch or dextrinized starch may be used as the adhesive with the improved gypsum pigment instead of casein, if desired.

The equipment required for making the defiocculated gypsum, is an agitator of suflicient strength that a positive agitation or kneading will be given to the whole plaster-water mix, together with such accessories as storage tanks, piping, weighing and measuring devices, etc. Since plaster, mixed at the consistency required in this process when soda ash is used as the set inhibitor does show a distinct, though reduced, tendency to set during hydration, the agitator must be sufiiciently powerful to keep the slurry in fluid motion at all times. The maximum speed that can be used without throwing the material out of the agitator gives the best results. A power driven bakery mixer gives good results.

The defiocculated gypsum coating colors are handled on the coating machine in the usual manner. It may be found advisable to run the machine at a somewhat higher speed than usual, or else use a somewhat harder sized raw stock, as the paper stock seems to draw the water from the deflocculated gypsum coating faster during .the setting stage of the operation than it does from a clay coating. The defiocculated gypsum coatings are relatively more satisfactory in medium and heavy coatings than in light (below 10 pounds) coatings The essential point in finishing deflocculated gypsum coatings is to calender the paper with decidedly more pressure than is usually used, particularly, more than is used on satin white coatings. This can be safely done, as the deflocculated gypsum coating is substantially free from any tendency to blacken under pressure up to the limit the fiber will stand without crushing. The heavy calendering aids in bringing out the levelness characteristic of deflocculated gypsum coatings and in increasing the glare, and thus improves the printing qualities of the stock.

The main advantages of our improved defiocculated gypsum coating are:

Compared to clay coatings:

1. Smoother surface for printing. 2. Somewhat'higher glare. 3 Whiter color. Compared to satin white-clay coating:

1. Equal or better printing smoothness. 2. A more flexible sheet, permitting heavier coating without danger of cracking on folding. 3. Freedom from tendency to blacken on calendering. 4. Lower cost.

When a recrystallized gypsum paper coating pigment is to be prepared using a larger amount of water than that described in the examples above, the use of an electrolyte, such as sodium phthalate, is unnecessary since the mix is too thin to stiffen greatly during the hydration.

In this modified form of process, about 85 lbs. of calcined gypsum may be mixed with about 100 gallons of water in a ball mill for a period of about to minutes which is about 8 to 10 minutes after the calcined gypsum ordinarily stifiens due to hydration. The resulting recrystallized gypsum has an exceptionally fine crystal structure, and is therefore of especial smoothness in paper coating. After mixing, the slurry is passed through a centrifuge or filter press to remove most of the water. The moisture content of the thickened pulp is then determined, and the requisite quantity of clay, water, soda ash and casein glue are added as in the preceding mixing is carried out for about 40 minutes.

We'would state in conclusion that while the examples described constitute practical embodiments of our invention, we do not wish to limit ourselves precisely to these details, since manifestly, the same may be considerably varied without departing from the spirit of the invention as defined in the appended claims.

Having thus described our invention, we claim as new and desire to secure by Letters Patent:

1. A paper coating mixture comprising casein, a casein solvent, gamma gypsum, alkali metal phthalate, clay, formaldehyde, and 1 to 2 parts by weight of water based upon the "gamma yps 2. A paper coating mixture, comprising casein, a casein solvent, gamma gypsum," alkali metal phthalate, and 1 to 2 parts by weight of water based upon the "gamma gypsum."

I 3. A paper coating mixture, comprising casein, a casein solvent, gamma gypsum, alkali metal phthalate, formaldehyde, and 1 /2 to 2 parts by weight of water based upon the gamma gypsum."

4. A paper coating mixture comprising casein, a casein solvent, gamma gypsum," and water.

5. A paper coating mixture comprising casein, a casein solvent, formaldehyde, clay, "gamma gypsum, and water.

6. A paper coating mixture comprising a slurry 'of gypsum crystals suspended in an aqueous long as their transverse diameter and having an average length along the major axis of about 5.4 microns.

9. As an article of manufacture, paper having a surface coated with crystals of gamma gypsum" embedded in a formaldehyde-hardened casein film.

10. As an article of manufacture, paper having a surface coated with clay and crystals of gamma ypsum embedded in aformaldehyde-hardened casein adhesive film.

JAMES S. OFFUTT. JOSEPH W. GILL. 

